The Nazis actually did some "scientific experiments" in this vein, I think it was in Dachau. They'd keep captives suspended on a T-beam cross with arms stretched out and without a foot support (or any headplank: that's somehow like what the crosses of ancient Roman crucifixions oftne looked like). After a few hours the "research persons" started having cramps and troubled breathing, however, if they were given a foot support, the trouble petered out and they seemed stable once again.
The results of those experiments seemed to be, then, that a crucified /suspended person with arms stretched out and no food or drink, could well stay alive for days (this is attested from antiquity) if he had a foot support, but if he/she had none, then they'd probably be badly affected in less than 12 hours.